This invention relates to heating elements. In particular, it relates to a novel electrically conductive fibrous web which can provide a uniform heat over fairly large areas for sustained periods, and to a fire-resistant, sandwich-type heating element containing this web.
Heating elements capable of generating and sustaining moderate uniform temperatures over large areas are required for a variety of applications, ranging from underwater heaters to structural heating panels to heating pads and electric blankets for consumer use. Loops or grids of wire cannot provide such uniform temperatures; wires which are sufficiently fine and closely spaced to provide the required temperatures without "hot spots" are necessarily fragile and easily damaged, with the attendant dangers of fire and electrical shock. Metal sheet and foils are suitable only for the limited range of applications corrosion resistance is not required, and cost is no object. Because of the shortcomings of traditional metal wires and sheets, a great deal of effort has been devoted to developing woven and non-woven carbon fiber webs for use as heating elements. Typical nonwoven webs are described, for example, in U.S. Pat. No. 3,367,851 issued 2/6/68 to Filreis et al and in U.S. Pat. No. 3,774,219 issued 11/27/73 to Sato et al. These patents teach that short carbon fibers must be used in order to achieve a uniform sheet which will have the desired uniform heat dispersion properties. In Sato et al, "fibers of 5 to 20 microns in diameter and 3 and 10 mm or so in average fiber length are used. When the fiber length exceeds 10 mm, it becomes difficult from the technical standpoint to manufacture the electroconductive sheet containing uniformly dispersed carbon fiber therethroughout with the result that irregularity in the resistance value from place to place in the sheet becomes prohibitive" [Sat et al, col. 3, lines 20-27]. Filreis et al recommends the use of fibers having "an average fiber length from 1/16 to 7/16 inch, preferably from 1/8 to 3/8 inch. When the average fiber length falls below this lower limit the resistivity of the web at constant loading increases markedly, whereas average fiber lengths above the maximum [sic] difficult to handle and blend with the other fibers and can also tend to produce sheets with erratic resistivities from lot to lot. " [Filreis et al, col 3, lines 19-26]. There are a number of disadvantages, however, inherent in making non-woven conductive webs with short carbon fibers. Conductivity varies roughly as the square of fiber length in a non-woven; consequently, in order to obtain a given conductivity much higher percentages of shorter fibers must be used. Certain desireable mechanical properties, such as web strength and flexibility, also improve significantly with increased average fiber length. The necessity of loading the web with large quantities of short carbon fiber makes it difficult to produce acceptable physical properties in webs made on commercial machines. Finally, since uniform dispersion of the short carbon fibers is so critical to the electrical properties of the web, cellulose is the preferred non-conductive fiber; the non-conductive fiber content of the web must be selected with dispersability and compatibility with carbon fibers as the primary considerations. Papers and plastic sheets loaded with conductive particles, such as metal flakes and carbon black, have also been tried but the excessive loading requirements and poor mechanical properties are even more pronounced in these.